First you'll see a few diagrams that show the journey visually, which I find
really helps, and then you'll find a wonderful chart on the bottom
of this page that analyzes how Star Wars and The Matrix used
Campbell's monomyth.

Here's a simplified diagram:

Next up, a detailed diagram by Lisa A. Paltz Spindler:

Now one with great detail:

Finally, I love this gameboard-style depiction:

Next up, a nice analysis of two popular SF works and how they use Campbell's ideas, by the "Star Wars Origins" people.

Campbell

Star Wars

The Matrix

I: Departure

The call to adventure

Princess Leia's message

"Follow the white rabbit"

Refusal of the call

Must help with the harvest

Neo won't climb out window

Supernatural aid

Obi-wan rescues Luke from sandpeople

Trinity extracts the "bug" from Neo

Crossing the first threshold

Escaping Tatooine

Neo is taken out of the Matrix for the first time

The belly of the whale

Trash compactor

Torture room

II: Initiation

The road of trials

Lightsaber practice

Sparring with Morpheus

The meeting with the goddess

Princess Leia (wears white, in earlier scripts was a "sister" of a mystic order)

The Oracle

Temptation away from the true path

Luke is tempted by the Dark Side

Cypher (the failed messiah) is tempted by the world of comfortable illusions

Atonement with the Father

Darth and Luke reconcile

Neo rescues and comes to agree (that he's The One) with his father-figure, Morpheus

Apotheosis (becoming god-like)

Luke becomes a Jedi

Neo becomes The One

The ultimate boon

Death Star destroyed

Humanity's salvation now within reach

III: Return

Refusal of the return

"Luke, come on!" Luke wants to stay to avenge Obi-Wan

Neo fights agent instead of running

The magic flight

Millennium Falcon

"Jacking in"

Rescue from without

Han saves Luke from Darth

Trinity saves Neo from agents

Crossing the return threshold

Millennium Falcon destroys pursuing TIE fighters

Neo fights Agent Smith

Master of the two worlds

Victory ceremony

Neo's declares victory over machines in final phone call

Freedom to live

Rebellion is victorious over Empire

Humans are victorious over machines

Common Mythic Elements

Two Worlds (mundane and special)

Planetside vs. The Death Star

Reality vs. The Matrix

The Mentor

Obi-Wan Kenobi

Morpheus

The Oracle

Yoda

The Oracle

The Prophecy

Luke will overthrow the Emperor

Morpheus will find (and Trinity will fall for) "The One"

Failed Hero

Biggs

In an early version of the script, Morpheus once believed that Cypher was "The One"

WearingEnemy's Skin

Luke and Han wear stormtrooper outfits

Neo jumps into agent's skin

Shapeshifter (the Hero isn't sure if he can trust this character)

Han Solo

Cypher

Animal familiar

R2-D2, Chewbacca

N/A

Chasing a lone animal into the enchanted wood (the animal usually gets away)

Luke follows R2 into the Jundland Wastes; The Millennium Falcon follows a lone TIE fighter into range of the Death Star

Neo "follows the white rabbit" to the nightclub where he meets Trinity

And there it is, a pretty good introduction to Campbell's Hero's Journey; now
start using this to analyze your own work. I hope you find this useful!